1. Field of the Art
This invention relates to a method for manufacturing cross prisms, and more particularly to a method for manufacturing high precision cross prisms.
2. Prior Art
Cross prisms employing two kinds of optical coatings in crossed relation, more specifically, cross dichroic prisms employing mainly two kinds of dichroic coatings in crossed relation have been in use for separation or synthesis of light of blue, green and red wavelength components. Shown in FIG. 8 is an example of such crossed dichroic prisms. As shown in FIG. 8, a blue wavelength light ray modulated by a blue light modulation light valve 101B, a green wavelength light ray modulated by a green light modulation light valve 101G and a red wavelength ray modulated by a red light modulation light valve 101R are respectively fed to a crossed dichroic prism 100 from three different directions.
The crossed dichroic prism 100 is provided with, in crossed relation, a blue reflecting dichroic coating 104B with properties of reflecting only a blue wavelength component and a red reflecting dichroic coating 104R with properties of reflecting only a red wavelength component. Of the blue, green and red components which are incident on the cross dichroic prism 100, the green component is transmitted through the blue and red dichroic coatings 104B and 104R, while the blue component is reflected off on the blue reflecting dichroic coating 104B and the red component is reflected off on the red reflecting dichroic coating 104R. A color image is synthesized from these transmitted and reflected components and projected on a screen 103 through a projection lens 102.
In order to project color images accurately, the blue reflecting dichroic coatings 104B as well as the red reflecting dichroic coatings 104R of the crossed dichroic prism 100 should be deposited in one and same plane of high rectitude. For example, in the case of the crossed dichroic prism 100 of FIG. 8 which is composed of four triangular prisms 100A, 100B, 100C and 100D, the blue reflecting dichroic coating 104 or the red reflecting dichroic coating 104R is deposited on one of the two inclined sides of the four triangular prisms, prior to cementing the four prisms together. At the time of cementing the four triangular prisms to each other to form the dichroic prism 100, it is a paramount requisite to make sure that apexes of the respective triangular prisms meet together accurately at one point. Deviational errors in cemented apex portions, that is to say, in a center portion of the dichroic prism 100 can result in incorrect or false optical characteristics due to lack of plane rectitude. Especially, deviational errors in center portions of the cemented dichroic prism 100, where components of three colors are synthesized, can have great effects on projected images, giving rise to problems such as unmatched double images, blurring etc.
In this connection, disclosed in Japanese Laid-Open Patent Application 2003-43224 is a method of fabricating a crossed dichroic prism by preparing two sets of cemented glass blocks by joining two glass blocks of different shapes, joining the two cemented glass blocks in optically aligned state by way of sticking-out alignment planes on the two cemented glass blocks to obtain an integrated block structure, and cutting and polishing four corners of the integrated block structure.
More specifically, according to the above-mentioned Japanese Laid-Open Patent Application 2003-43224, first and third rectangular parallelepiped prisms having a square shape in cross section are employed in combination with second and fourth prisms which are longer than the first and third prisms at lateral sides perpendicular to a cross-sectional surface. After polishing the respective prisms, a first dichroic coating is deposited on lateral sides of the first and third prisms. The first and third prisms are temporarily bonded to the second and third prisms, respectively, in such a way as to form a sticking-out reference plane on each one of the first and third prisms. Then, two cemented blocks are formed by curing an adhesive agent, securing plane rectitude of the first dichroic coating with the use of a reference member, and a second dichroic coating is deposited on a lateral side of each cemented block. In the next place, after temporary bondage, the two cemented blocks are adjusted relative to each other by the use of a temporary adhesion reference member such that the sticking-out reference planes are brought to on one and same plane, and the adhesive agent is finally cured to obtain an integrated block structure. A crossed dichroic prism is fabricated out of the integrated block structure by cutting and polishing four ends of the latter.
In the case of the above-mentioned Japanese Laid-Open Patent Application 2003-43224, even if first and second dichroic coatings are deposited on the integrated block structure, there is still a problem that due to concentration of stress distortions occur to center portions of the crossed dichroic prism at the time of cutting and grinding four corners of the integrated block structure in a later stage. Namely, in a cutting stage, forces are applied to the integrated block structure in the cutting direction, including components which are applied in a direction parallel with the bonded surface on which the first dichroic coating is formed, and component which are applied in a direction parallel with the joined plane on which the second dichroic coating is formed. Further, forces are applied toward the center of the integrated block structure in a grinding stage, including components acting in a direction parallel with the joined plane on which the first dichroic coating is deposited, as well as components acting in a direction parallel with the joined plane on which the second dichroic coating is formed.
In this way, at the time of cutting and grinding an integrated block structure, forces are applied not only in a direction parallel with the joined plane on which the first dichroic coating is deposited, but also in a direction parallel with the joined plane on which the second dichroic coating is deposited. Therefore, even if the first and second dichroic coatings are deposited on the integrated block structure satisfactorily in plane of high rectitude, it is likely that they are stressed under the influence of concentration of stress in center portions of the integrated block structure in cutting and polishing stages, and deviations of the dichroic coatings when stressed to an extremely large degree. Especially in a grinding stage, strong forces are applied toward the center of an integrated block structure so that in some case raised portions are developed in center portions of the first and second dichroic coatings, causing a complete breakage to center portions of the dichroic coatings in some cases. As a consequence, it becomes difficult to get correct optical performances in center portions of a crossed dichroic prism which play the most important role.
On the other hand, attempts have been made to cut down the grinding force to a moderate level. However, a moderate grinding process is time consuming and inferior in yield.